1. Field of the Invention
The present invention relates to a control valve suitable for controlling a discharging capacity of a variable displacement compressor.
2. Description of the Related Art
An automotive air conditioner generally includes a compressor, a condenser, an expander, an evaporator, and so forth. The compressor compresses a refrigerant flowing through a refrigerant cycle of the air conditioner and then discharges the thus compressed refrigerant as a higher-temperature and high-pressure gas refrigerant. The condenser condenses the gas refrigerant. The expander adiabatically expands the condensed liquid refrigerant so as to produce a low-temperature and low-pressure refrigerant. Then the evaporator evaporates the low-temperature and low-pressure refrigerant and thereby subjects the thus evaporated refrigerant to a heat exchange with the air inside the vehicle. The refrigerant evaporated by the evaporator is again brought back to the compressor, which in turn circulates through the refrigerant cycle.
Used as such a compressor as described above is a variable displacement compressor (hereinafter referred to simply as “compressor” also) capable of controlling the refrigerant discharging capacity in order to maintain a constant level of cooling capacity irrespective of the engine speed. This compressor has a piston for compression linked to a wobble plate that is mounted to a rotational shaft rotatingly driven by an engine. And the compressor controls the refrigerant discharge rate by changing the stroke of the piston through changes in the angle of the wobble plate. The angle of the wobble plate can be changed continuously by changing the balance of pressure working on both faces of the piston as part of the discharged refrigerant is introduced into an airtight crankcase. The pressure within this crankcase (hereinafter referred to as “crank pressure”) Pc is controlled by a control valve for a variable displacement compressor (hereinafter referred to simply as “control valve” also), which is provided between the discharge chamber of the compressor and the crankcase or between the crankcase and the suction chamber thereof.
One of these control valves, such as one disclosed in Reference (1) in the following Related Art List, controls the crank pressure Pc by adjusting the amount of refrigerant introduced into the crankcase in accordance with a suction pressure Ps, for instance. This control valve includes, for example, a pressure-sensing section to develop a displacement by sensing the suction pressure Ps, a valve section for controlling the opening and closing of the passage from the discharge chamber to the crankcase in response to a drive force from the pressure-sensing section, and a solenoid capable of changing the setting value of the drive force at the pressure-sensing section by external electric current. The control valve like this opens and closes the valve section in such a manner as to maintain the suction pressure Ps at a pressure set by the external electric current. Generally, the suction pressure Ps is proportional to a refrigerant temperature at the exit of the evaporator, and thus the freezing or the like of the evaporator can be prevented by maintaining a set pressure at or above a predetermined value. Also, when the engine load of a vehicle is high, the compressor can be operated at the minimum capacity by fully opening the valve section with the solenoid turned off and by setting the wobble plate substantially at a right angle to the rotational shaft with the crank pressure Pc set high.
To accurately control the crank pressure Pc, such a control valve as described above is often provided with a seal structure by which to prevent the refrigerant from leaking from a high pressure side to a low pressure side at places excluding the valve section. In the structure described in Reference (1) in the following Related Art List, for example, a portion where a valve element is formed integrally with a rod and operates, namely, a guide hole joining a discharge chamber communicating port, which communicates to a discharge chamber, to a suction chamber communication port, which communicates with a suction chamber, functions as a sliding portion of a rod. And an O-ring is provided on the outer periphery of the rod. This structure can prevent a high-pressure refrigerant introduced from the discharge chamber communicating port from leaking into a low pressure chamber communicating with the suction chamber communicating port.